Or upgradeable to do the same. For that matter, even if both Galaxy and Intrepid have Type X phasers, capable of outputting X terawatts (or whatever) of power, who's to say the Intrepid has that kind of juice to dump into the phasers anyway? The Galaxy may have more raw power to use for phasers or shields or whatever. Same speakers in the system, more juice in the bigger D batteries, if you'll pardon the pun.
hehe,but the Intrepid has a mark 9 warpcore that allows it to achieve a speed of 9.975 impossible to for the Galaxy to reach,if you remember warp 9.6 was max speed 9.8 could destroy the ship,i highly sought a ship with slower main propulsion has more power.
In addition the Galaxy Class was less military,stated by the producers themselves on a youtube video i have seen,they also said that the Intrepid was more combat oriented.
another thing to think about is that the Galaxy class has civilians families,schools,18 holodecks,large Saloons,Lecture halls,many Gyms etc...all of these cost a lot of power...